FIG. 1 depicts a conventional betatron driving circuit. A high voltage D.C. power supply 10 is coupled across a capacitor 12 to modulator (pulse-generating) circuits 14, which pulse the primary betatron coil circuits 16 at the desired repetition rate with time-varying voltage pulses to accelerate electrons trapped within betatron field. During each acceleration cycle, the energy stored in the capacitor 12 is transferred to the betatron magnetic circuit through a switching network (contained in the modulator 14), and at the end of each cycle the remaining energy in the magnetic circuit is returned to the capacitor 12 through a recovery network (also in the modulator 14). Due to losses in the magnetic circuit, the capacitor and the switching/recovery networks, a fraction of the original energy is not recovered and must be replenished by the power supply. That means that the power supply 10 must have an output voltage greater than, or at least equal to, the maximum voltage intended for the capacitor 12 or, in other words, the maximum excitation voltage to be applied to the betatron circuits.
In a typical borehole logging tool, such maximum voltage would be on the order of 1-2 KV or possibly even higher. Although not a problem in a laboratory environment, the requirement for a high voltage capacitor/charging power supply system is both troublesome and costly in the hostile, confined environment of a borehole logging tool.
A pulse-generating circuit for a betatron having an independent low voltage power supply has been described by Baginskii et al. for use in flaw detection under nonstationary conditions, "Thyristor Current-Pulse Generator for Betatron Electromagnet with Independent Low-Voltage Supply", Nuclear Experimental Techniques, Vol. 31, No. 4, Part I, pp. 832-835, 1989, translated from Pribory i Tekhnika Eksperimenta, No. 4, pp. 16-18, July-August, 1988. The Baginskii et al. circuit employs a conventional rectifier bridge, the D.C. diagonal of which contains the betatron winding and the A.C. diagonal of which contains a high voltage storage capacitor for supplying the high voltage excitation energy to the betatron. Energy to replenish the losses in the circuit is transferred from the low voltage power supply to the high voltage storage capacitor via an intermediate inductive storage circuit. This circuit is reported to have produced a voltage of 1.5 KV on the betatron winding using a 27 V D.C. power supply. Although an inductive storage device can act as an effective buffer between the low voltage power supply and the high voltage storage capacitor, such circuits are subject to excess energy losses. This is because the energy is stored in the magnetic field supported by the current flowing in the inductor. Thus energy is being lost while it is stored in the inductor due to the non-zero current.